The present invention relates to an art of detecting and controlling an optical pickup actuator for tracking control for optical disks, and more particularly to an actuator position detector, an actuator position controller, and a track search controller for use with such optical disks.
Heretofore, various tracking control methods have been developed for use with read-only optical disks such as compact discs. Generally, it has been customary to detect whether a beam spot for reading recorded information is present on the center axis of a recording track of the optical memory disk according to the three beam method or the radial push-pull method (spot-on-track detection).
In the three-beam method, two satellite spots, i.e., leading and trailing spots, are projected onto the optical disc at positions forward and rearward, respectively, of a main scanning spot for reading recorded information from a recording track. The leading and trailing spots are spaced radially from each other by a certain offset in a direction normal to the track direction, i.e., the axis (center line) of the recording track, along which the main scanning spot travels. Reflected beams from the leading and trailing spots on the optical disk are detected by respective photodetectors, and the difference between photoelectrically converted output signals from the photodetectors is calculated and produced as a differential output signal or tracking error signal. When the main scanning spot exists on the axis of the recording track, the differential output signal is of a zero value. However, when the main scanning spot exists off the axis of the recording track, the differential output signal is not of a zero value but of a positive or negative value. According to a tracking servo control process, an actuator actuates an objective lens of an optical pickup to control the position of the spots on the optical disk so that the differential output signal becomes zero. In a track jump control process, the position of the spots can also be controlled by counting zero crossings where the differential output signal becomes zero.
The spot-on-track detection is possible according to the three-beam method insofar as the optical memory disk is a read-only optical disk. More specifically, as shown in FIG. 12 of the accompanying drawings, the read-only optical disk has recording tracks (shown blank) that are composed of pits representing information signals, and the intensity of light reflected from these pits is less than the intensity of light reflected from mirror-finish areas (shown stippled) other than the recording tracks. That is, there is a radial contrast, i.e., the difference between the intensity of light reflected from tracks or grooves and the intensity of light reflected from intertrack areas or lands, between the recording tracks and the intertrack mirror finish areas. Consequently, the differential output signal is not zero if the main scanning spot is off track.
There are known WORM (Write Once Read Many) optical memory disks and E-DRAW (Erasable Direct Read After Write) optical disks. These optical disks have a non-recorded region that includes tracks (often referred to as grooves) where information is to be recorded subsequently and intertrack areas other than the tracks. In the non-recorded region, the intensity of light reflected from these tracks and the intensity of light reflected from the intertrack areas are almost the same as each other, i.e., there is no substantial radial contrast between the tracks and the intertrack areas. Therefore, it is highly difficult to effect the spot-on-track detection in the non-recorded region according to the three beam method since the differential output signal is zero regardless of whether the main scanning spot is on track or off track.
The radial push-pull method, which is effective to carry out the spot-on-track detection on such WORM and E-DRAW optical memory disks, employs a two-segment photodetector or two photodetector halves PD as shown in FIG. 1(A). When a light beam is applied as a scanning spot to an optical memory disk, it is reflected as light of a zeroth-order L.sub.0 and light of positive and negative first orders L.sub.+1, L.sub.-1 due to an irregular disk surface configuration that is composed of a recording track (groove) TR and an intertrack area other than the recording track TR. The reflected light is applied to the two photodetector halves PD in three areas, i.e., the first area S.sub.0 where the zeroth-order light is applied, the second area S.sub.+1 where the zeroth-order light L.sub.0 and the positive first order light L.sub.+1 are applied as diffracted, and the third area S.sub.-1 where the zeroth-order light L.sub.0 and the negative-first-order light L.sub.-1 are applied as diffracted.
The two photodetector halves PD have respective photodetector surfaces A, B whose output terminals are connected to the respective input terminals of a subtracter, which produces a differential output signal or tracking error signal representative of the difference between output signals from the photodetector halves PD. When the scanning spot is on track, the intensity of light applied to the second area S.sub.+1 and the intensity of light applied to the third area S.sub.-1 are equal to each other, and hence the differential output signal is zero. When the scanning spot is off track, the intensity of light applied to the second area S.sub.+1 and the intensity of light applied to the third area S.sub.-1 are different from each other, and hence the differential output signal is of a positive or negative value. Consequently, the push-pull method is capable of carrying out the spot-on--track detection.
However, if the optical memory disk is radially inclined or the lens of the optical system in the optical pickup is displaced off an optical axis, then a light spot offset .DELTA.I (see FIG. I(B)) occurs on the photodetector according to the push-pull method. With the light spot offset .DELTA.I, even when the scanning spot is right on the axis of the recording track TR, the produced tracking error signal is of a value 2.DELTA.I (FIG. 2), but not zero, and the tracking servo control process is not properly performed.
It has been proposed in U.S. patent application Ser. No. 779,013 to employ three light beams, determine push-pull differential output signals from these three light beams, and process the three push-pull differential output signals for affecting a good tracking servo control process in the non-recorded region of an optical disk while removing the effect of the offset.
According to the conventional tracking servo control arrangement, the actuator for controlling the position of an objective lens is fixedly mounted on an optical pickup by a spring or the like, and the optical pickup is supported on a carriage for movement in the radial direction of the optical disk. When the optical pickup is moved for a track jump such as in a high speed search mode, the actuator tends to vibrate in the radial direction of the optical disk as shown in FIG. 3. At this time, the actuator moves with a vibration along a path P.sub.A. The actuator vibrates primarily at a constant frequency, known as a minimum resonant frequency f.sub.0, usually in the range of from 10 to 100 Hz.
When the actuator thus vibrates, more zero crossings are counted than they actually are, and the track jump is finished before the optical pickup reaches a desired track position. Accordingly, the time required to search for a desired track is relatively long. It has been customary to cope with the vibration of tile actuator with a sensor for detecting the position of the objective lens. However, the prior solution has proven unsatisfactory in that the optical pickup and the track search mechanism are complex in structure and large in size, and the sensor has to be provided as a dedicated sensor.